This invention relates to a gas turbine apparatus, and more particularly to a type thereof wherein the contents of nitrogen oxides (NO.sub.x) and carbon monoxide (CO) in the exhaust gas are reduced by use of a so-called premixed fuel combustion method.
A conventional as turbine apparatus shown in FIG. 1, wherein an air compressor 2 compresses air 1 supplied from outside to a high-pressure, and the air 3 thus compressed is introduced into an annular chamber 5 surrounding a combustion cylinder 6, the chamber 5 and cylinder 6 constituting a combustion device 4. The high-pressure air cools the combustion cylinder 6 while it circulates through the annular chamber 5, and thereafter is introduced into the cylinder 6 through air supply ports 7 and a swirler 8.
Fuel supplied from a fuel supply (source, not) shown, is introduced through a fuel supply system 10 having a regulating valve 9, into the combustion cylinder 6 of the combustion device 4. The fuel thus introduced is injected into a reverse-flow region 12 within the cylinder 6 through one or more nozzles 11, and is ignited by an igniting device 13. After a continuous combustion of an amount of fuel at a constant pressure, a high temperature gas 14 is produced, and the gas 14 is supplied into a gas turbine 15 for operating the same. The driving force thus generated from the gas turbine 15 is partly consumed by the air compressor 2, while most part of the driving force is consumed for driving a driven device 16 such as a dynamo. The amount of fuel supplied to the combustion device 4 is controlled by controlling the opening of the regulating valve 9 provided in the fuel supply system 10 according to the load required for the driven device 16.
Regardless of the number of nozzles 11 provided for the cylinder 6, a large amount of so-called thermal NO.sub.x is ordinarily contained in the high temperature gas 14. By selecting a suitable number of nozzles and configuration of the ports 7, the temperature of the flame produced in the combustion cylinder 6 can be locally reduced, so that the amount of NO.sub.x thereby produced is somewhat reduced.
It has been found that a premixed fuel combustion method is advantageous for reducing flame temperature in the combustion cylinder 6. A relationship between NO.sub.x production and the mixing ratio of fuel to air is shown in FIG. 2. When the premixed gas combustion method is not used, the amount of NO.sub.x contained in the high temperature gas 14 increases exponentially as indicated by a curve 20 according to an increase in the fuel/air mixing ratio. On the other hand, when the premixed gas combustion is used, the amount of NO.sub.x is restricted as indicated by the curves 21a.about.21d regardless of an increase in the fuel/air ratio, thus leading to a remarkable reduction of NO.sub.x, and this effect appears more significant when the mixing ratio increases to 50.about.80% (as shown by 21a.about.21d).
A conventional gas turbine utilizing the premixed fuel combustion is shown in FIG. 3. In this construction, fuel supplying systems 30 and 31 are provided, in which the system 30 is connected to the nozzle 11 so that the fuel is supplied directly into the combustion cylinder 6, while the system 31 supplies fuel to a premixing chamber 32 provided around the combustion cylinder 6, in which high-pressure air 3 has been supplied from the air compressor 2. The fuel thus supplied is mixed with high-pressure air 3, and the mixed gas 36 is introduced into the combustion cylinder 6 through the air supplying ports 7. The mixed gas 36 is ignited by a high-temperature gas produced by the fuel supplied from the fuel supplying system 30.
The contents of NO.sub.x and Co in the exhaust gas from the gas turbine 15 are varied as shown in FIG. 4 according to the variation in the turbine load. When the turbine load is a rated value (100%), both of the characteristic curve 37 showing the content of NO.sub.x and the characteristic curve 38 showing the content of CO are below the regulatory limits 39 and 40. However, when the load of the turbine is smaller than the rated value, being approximately equal to a transfer point C where the supply of fuel is transferred from the system 30 to the system 31, the contents of NO.sub.x and CO widely exceed the upper limits of the regulation. In particular, the content of CO increases at this point abruptly, thereby reducing the combustion efficiency of the combustion device 4, and the plant efficiency of the turbine apparatus is reduced. Furthermore, in a reduced load operation, a combustion vibration tends to occur in addition to the reduction of the combustion efficiency, and the reliability of the gas turbine is thereby reduced.
To alleviate this difficulty, an air by-pass conduit 34 having an air regulating valve 35 has been provided between the output of the air compressor 2 and the output side of the combustion cylinder 6. That is, by controlling the opening of the air regulating valve 35, the amount of the high-pressure air 3 supplied into the combustion cylinder 6 is controlled, and the fuel/air mixing ratio in the high-temperature gas 14 is maintained substantially constant, thereby reducing the NO.sub.x and CO densities in the exhaust gas.
In accordance with the above described procedure, the maximum contents of NO.sub.x and CO can be reduced less than the regulation limits 39 and 40. However, since the high-pressure air 3 of a comparatively low temperature is supplied into the high-temperature gas 14, such a procedure is not advantageous in view of the temperature distribution of the apparatus, and various problems tend to occur in the construction of the gas turbine.